There are generally two forms of tungsten carbide; monotungsten carbide (WC) and ditungsten carbide (W.sub.2 C). It is well-known that WC is useful in the manufacture of commercially worthwhile items, such as cutting tools, dies and drilling tools, whereas W.sub.2 C generally is not. In fact, W.sub.2 C degrades the properties, such as strength of WC objects, even when present only in small quantities.
In producing said WC items, it is common for a tungsten carbide powder to be combined with a metal, such as cobalt and, subsequently, densified into a WC/Co cemented carbide by heating. The heating may take place at a pressure ranging from vacuum to pressures greater than atmospheric pressure.
In a cemented carbide part, the tungsten carbide, grain size, grain size distribution and grain chemistry greatly influence the final part properties. As already stated above, W.sub.2 C should be avoided when making cemented tungsten carbide parts. Generally, smaller grain size in a cemented part results in improved strength. In addition, smaller grain sizes often result in higher hardness at a given cobalt addition. Non-uniformity of grain size in a cemented tungsten carbide part adversely affects the strength of and the surface condition of the part after grinding. The non-uniformity of grain size in the cemented WC part is primarily due to exaggerated grain growth during the densification of the part. The grain growth can be controlled by addition of grain growth inhibitors, such as VC, Cr.sub.3 C.sub.2 or TaC, or starting with a WC powder having as narrow (i.e., uniform) as possible particle size distribution.
WC powder, which has an average particle size less than 0.2 to 0.3 micrometer, can cause exaggerated grain growth due to the increased reactivity associated with the fine particle size. It has also been reported that standard grain growth inhibitors, as described above, are not effective when sintering a cemented WC part using said fine WC powder. The critical parameter to sinter said fine WC powders was reported to be the WC powder grain size distribution (Suzuki et al, J. Jap. Soc. Powder and Powder Met., Vol. 19, p. 106-112, 1972). Thus, it is desirable to be able to increase the particle size or control the particle size distribution of very fine WC powder (less than 0.2 to 0.3 micrometer) to reduce the possibility of grain growth during the densification of a cemented WC part.
Typically, monotungsten carbide is formed by the carburization of tungsten metal. The basic process steps commonly are:
(a) calcining of ammonium paratungstate or tungstic acid to one of the stable forms of tungsten oxide, such as WO.sub.3, WO.sub.2.multidot.83, WO.sub.2.multidot.65 and WO.sub.2, PA0 (b) reducing the tungsten oxide to tungsten metal powder in hydrogen, PA0 (c) mixing the tungsten metal powder with a powdered form of carbon and PA0 (d) carburizing the tungsten and carbon mixture at a temperature in excess of 1100.degree. C. in a reducing (hydrogen containing) atmosphere. PA0 (1) depth of powder bed during reduction, PA0 (2) flow rate of hydrogen, PA0 (3) dew point of the hydrogen gas and PA0 (4) reduction temperature. PA0 a) forming a carbon-precursor mixture by mixing a precursor, comprised of (i) a transition metal oxide and (ii) one or more materials selected from the group consisting of: a transition metal carbide; a transition metal and a substoichiometric transition metal carbide, in the presence of a source of carbon in an amount sufficient to form a reduced mixture in step (b), the reduced mixture comprised of the transition metal carbide and substoichiometric transition metal carbide, wherein the amount of the transition metal oxide and transition metal is essentially zero, PA0 b) heating the carbon-precursor mixture under a reducing atmosphere to a reducing temperature for a time sufficient to produce the reduced mixture, PA0 c) forming a milled reduced mixture by milling the reduced mixture in the presence of a source of carbon sufficient to carburize the substoichiometric transition metal carbide in step (d) to form the transition metal carbide and PA0 d) heating the milled reduced mixture in a reducing atmosphere to a carburizing temperature that is greater than the reducing temperature for a time sufficient to carburize the substoichiometric transition metal carbide to form the transition metal carbide. PA0 (1) tungsten; PA0 (2) tungsten, ditungsten carbide and monotungsten carbide or PA0 (3) tungsten, ditungsten carbide, monotungsten carbide and carbon.
The resultant WC particle size is controlled by the size of the W metal powder formed in the above step (b). Tungsten metal particle size, as described by U.S. Pat. No. 3,850,614, is controlled mainly by:
Smaller particle size tungsten powder is produced by increasing gas flow, decreasing bed depth, reducing the dew point of the hydrogen gas and decreasing reduction temperature. By reducing the bed depth and reducing the temperature, the amount of tungsten powder that can be carburized to WC in a given period of time is decreased. The mechanism of growth has been attributed to a volatile WOH species directly associated with the water concentration in the gaseous environment (U.S. Pat. No. 3,850,614). Processes requiring the carburization of tungsten metal to form monotungsten carbide are, typically, limited to producing WC powder having a particle size of about 0.8 micron or larger because of the difficulty in producing W metal much smaller than this size due to, for example, the pyrophoric nature of such a fine tungsten metal powder. Because of the high hardness of WC, it is also difficult to grind WC to this small particle size. Even if WC were easily ground to the fine particle size, the grinding process inherently produces a wide particle size distribution compared to a controlled synthesis process.
Other methods of producing monotungsten carbide include the following methods. Steiger (U.S. Pat. No. 3,848,062) describes reacting a volatile tungsten species, such as WCl.sub.5, WCl.sub.4, WCl.sub.2, WO.sub.2 Cl.sub.2, WOCl.sub.4, WOF.sub.4 and W(CO).sub.6, with a vaporous carbon source, such as a volatile hydrocarbon or halogenated hydrocarbon. The vaporous carbon source is present in a quantity at least equal to WC stoichiometry during the above vapor phase reaction. The product from this reaction, a mixture of WC, W.sub.2 C and carbon, is then calcined at a temperature of about 1000.degree. C. for about 1 to 2 hours in hydrogen resulting in monotungsten carbide substantially free of ditungsten carbide.
Miyake (U.S. Pat. No. 4,008,090) describes a process having a first step of reacting a tungsten oxide with a carbon powder in a non-reducing atmosphere at a temperature greater than 1000.degree. C., thereby removing the oxygen, and a second step of reacting the product of the first step at a temperature higher than the first step in hydrogen to produce monotungsten carbide. Miyake specifies that the temperature must be greater than 1000.degree. C. in the first step to remove the oxygen. The removal of oxygen is necessary to avoid the reaction of hydrogen with oxygen forming water vapor which, consequently, reacts with carbon forming a volatile carbon-oxygen species, which causes the increase in particle size and non-uniform carbon content of the second step product (i.e., desired monotungsten carbide).
Kimmel (U.S. Pat. No. 4,664,899) describes a method to form monotungsten carbide comprising mixing tungsten oxide or ammonium paratungstate with carbon powder to form a resulting mixture, reducing said mixture in a non-reducing atmosphere, as Miyake does, for a sufficient time at a suitable temperature to produce resulting reduced mixture comprising tungsten, ditungsten carbide and monotungsten carbide, said reducing being carried out in the presence of sufficient carbon to produce a carbon content of less than 6.13 percent by weight in said resulting reduced mixture. Kimmel then describes determining the carbon content of said resulting reduced mixture, adding sufficient carbon to said resulting reduced mixture to increase the carbon content to at least the stoichiometric amount needed to form monotungsten carbide and carburizing in a hydrogen atmosphere the adjusted reduced mixture to form monotungsten carbide. Kimmel further describes that the product of the reducing of the tungsten oxide is a mixture of W, W.sub.2 C, WC and free carbon and that all of the oxide is reduced.
To make monotungsten carbide, these processes require either the slow process of complete reduction of a tungsten compound, such as tungsten oxide to tungsten metal in a hydrogen containing atmosphere, or the slow process of reduction of a tungsten compound to a mixture of tungsten metal, carbides of tungsten and free carbon in a non-reducing atmosphere (i.e., free of hydrogen). The tungsten or mixture is substantially free of oxygen (i.e., tungsten oxide) before finally carburizing in a reducing atmosphere to form monotungsten carbide. The oxygen is essentially completely removed to avoid particle growth due to formation of species, such as WOH, and the volatile loss of carbon by oxidation or hydrolysis during the carburization of the tungsten or mixture in a hydrogen-containing atmosphere. The removal of carbon during the carburization causes non-uniform carbon contents of the resultant carbide product (i.e., W.sub.2 C in the product). Manufacture of fine WC powders with uniform carbon contents are particularly a problem in industrial processes because of the large volumes of material processed which exacerbates the aforementioned problems.
Therefore, it would be desirable to provide a rapid industrial method to produce monotungsten carbide (WC) of uniform carbon content and small particle size that avoids the aforementioned problems.